1. Field of the Invention
The present invention relates to an automatic transmission of electric control type and, more particularly, to the hydraulic control of a hydraulic control system for improving the shift feel of the automatic transmission at the time of a shift.
2. Description of the Prior Art
In the automatic transmission of the prior art, the line pressure, i.e., the oil pressure to be fed to shifting components of the transmission such as clutches or brakes is set according to the throttle opening, the gear ratio and the vehicle speed.
Since, however, the oil pressure during a shift is at a constant level, it is difficult to improve the shift feel.
FIG. 10 is a diagram showing the shifting characteristics of an example in which the oil pressure is set to control the shift feel as well as possible while retaining the durability of the engage components of the prior art.
As indicated at (a) in FIG. 10, the electronic control means decides an upshift from the 1st to 2nd speeds according to the input conditions including the throttle opening and the vehicle speed and switches a signal to the shift solenoid from the 1st to 2nd speeds so as to effect the upshift. Simultaneously with this, as indicated at (b) in FIG. 10, the line pressure of the transmission (T/M) is switched from that for the 1st speed to that for the 2nd speed. Moreover, the engage pressure is also raised, as indicated at (c) in FIG. 10, to start the shift, and the T/M input r.p.m. (=the engine r.p.m.) drops from the shift starting point and rises at the shift end, as indicated at (d) in FIG. 10. On the other hand, the output shaft torque during the shift changes, as indicated at (e) in FIG. 10. What invites trouble here is the torque fluctuations a and b which occur at the shift starting point (i.e., the engage starting point of the engage components) and the shift ending point which detract from the feel of the shift.
In order to reduce the aforementioned torque fluctuations, therefore, it is generally known to drop the oil pressure to be applied to the engage components, as will be exemplified in the following.
FIG. 11 is a diagram of the shifting characteristics showing an example in which the set oil pressure is dropped to improve the shift feel.
As indicated at (b) in FIG. 11, similar to FIG. 10, the line pressure is dropped from the aforementioned line pressure for the 2nd speed (that is, the engage pressure slowly changes, as indicated at (c) in FIG. 11, and the T/M input r.p.m. changes, as indicated at (d) in FIG. 11). Then, the fluctuations of the output shaft torque at the shift starting/ending points are reduced to improve the shift feel. As indicated at (e) in FIG. 11, however, shift time t.sub.m2 is seriously lengthened. This means that the slip time of the engage components such as the clutches or brakes is prolonged to the detriment of the durability of the engagement components.